LANs for communication between various types of computing devices are well known in the art. Such computing devices are connected to the LAN through bus interface units (BIUs), the BIUs operating to transmit and receive information packets for the computing device(s) connected thereto. Protocols of various types, well known in the art, are implemented to enable the various BIUs to access the LAN, in an expedient and efficient manner, for transmitting information packets thereon.
It is therefore a desirable feature of such protocols that they serve to minimize the delay incurred by a BIU in waiting for access to the transmission medium in order to transmit its information packet. The protocol should also serve to maximize the bandwidth of the LAN, i.e. maximize the number of information packets successfully transmitted per unit time.
A contention bus accessing protocol, well known in the art, is one in which, generally, each BIU gains access to the transmission medium of the LAN, in order transmit its information packet, any time the subject BIU determines the medium is idle. When more than one BIU attempts to gain access to the transmission medium, a collision between information packets occurs and each BIU generally discontinues its transmission for a randomly selected time interval, thereafter reattempting to gain access to the medium. Examples of such a control structure are found in LANs following IEEE 802.3 and the Ethernet (a trademark of Xerox Corporation) specifications for a carrier-sense multiple-access (CSMA/CD) protocol. Such protocols generally incur increased delays when many BIUs have information packets to transmit, which results in many collisions occurring. Further, the loss of all information packets involved in each collision has an adverse impact on transmission medium bandwidth. Additionally, since medium access after each collision is determined by a random time factor, it is impossible to establish an upper bound on the interval between times when a BIU can gain access to the medium.
One contention protocol devised to overcome the above described shortcomings is the Preemptive BID (P-BID) Communication system, disclosed in U.S. Pat. No. 4,517,670 and incorporated in its entirety herein by reference. The P-BID protocol is implemented in a bus communication system comprising a transmit bus and a receive bus coupled together through a head-end amplifier. The BIUs are connected in parallel between the buses, each BIU being adapted to detect the presence of signals on the receive bus and signals originating from an upstream direction, away from the head-end, on the transmit bus. In accordance with the protocol, each BIU in a free mode, having an information packet to transmit, monitors the transmit and receive buses of the LAN and if both buses are idle commences transmission of its information packet. If, however, either bus is busy, e.g. an information packet being transmitted by another BIU is detected, the free mode BIU waits until the end of that packet is detected on the receive bus. It then initiates measurement of a communication period having a duration preferably on the order of the two way propagation delay (2T) of the transmission medium. The length of the communication period is selected to assure all BIUs have the opportunity to detect transmissions by all other BIUs in the communication system before gaining access to the transmit bus. If, by the end of the communication period no signals have been detected on the transmit or receive buses, the free mode BIU commences transmission. While transmitting, if the BIU detects upstream originating signals on the transmit bus, it aborts transmission and enters a preempted mode.
Following successful packet transmission by a most upstream transmitting BIU, the preempted mode BIUs detect the end of that successfully transmitted packet, thereupon commencing measurement of a communication period during which each preempted mode BIU transmits a beep. The most upstream preempted mode BIU detects no upstream originating beep and gains access to the transmit bus. BIUs not in the preempted mode upon detecting the beep(s) during the communication period, determine that preempted mode BIUs are attempting to gain access to the bus and defer to those BIUs. Each transmission by a preempted mode BIU is followed by a communication period until all such BIUs have transmitted their information packets. Thus as described above and in the above incorporated U.S. Pat. No. 4,517,670, operation of the P-BID protocol requires at least one communication period following every information packet transmission in order to apprise all BIUs in the system whether the information packets being received were transmitted by BIUs in the free or preempted mode. This need for communication periods can, however, degrade communication system performance at high bit rates. As the hardware available for use in LANs improves and transmission bit rates increase, the length of time required for a BIU to transmit an information packet decreases with respect to the length of the communication period. Since the speed of light is a constant, the two-way propagation delay of the transmission medium remains constant. Therefore, as the bit rate increases and the communication period remains constant in duration, the usable portion of the transmission medium bandwidth decreases. It is therefore desirable to provide a bus accessing protocol with the desirable features of the P-BID protocol in which the adverse effect of the communication period is minimized.
The communication system known as PRENET, described in the paper entitled "A Fiber Optic Contention Bus With Bounded Delays" by M.E. Ulug, IEEE Journal on Selected Areas of Communications, Vol. SAC-3, No. 6, November 1985, incorporated in its entirety herein by reference, when implemented in a two cable configuration, practices the P-BID protocol. In that form of PRENET, each station or BIU includes regenerators through which the transmit and receive buses are respectively looped. Each BIU further includes an energy detector such as provided in the P-BID system, for detecting transmit bus signals originating from an upstream direction away from the head-end which couples the transmit and receive buses together. The PRENET system suffers the same adverse effect on bandwidth caused by the requisite communication periods.
It is therefore an object of the present invention to provide a bus accessing protocol including the desirable features of the P-BID protocol, in which the adverse effect of the communication periods is minimized and that can be implemented in the PRENET two cable system.